1. Field of the Invention
The invention relates to a method and an apparatus for processing a motor signal of a DC motor, with a controllable frequency filter, which generates a speed-proportional output signal using the ripple contained in the armature current of the DC motor.
2. Description of the Related Art
A DC motor, which is also referred to as a commutator motor, is a permanent magnet electric motor, whose motor or armature current, as a result of the commutation, contains an AC component, also referred to as current ripple, which is superimposed on a DC component. Such a DC motor is in particular also used in an adjustment drive, for example in a window winder drive, a sunroof drive or a tailgate drive, of a motor vehicle. In this application, it is particularly important to enable, using simple means, reliable determination of the motor or armature position and therefore precise position determination of the motor-driven adjustment part (actuating element).
The frequency of the current ripple is dependent on the number of commutator or collector laminates of the motor armature. Therefore, it is possible to draw a conclusion, on the basis of the number of current ripple elements in a time interval, both on the rotary position of the motor, and therefore on the position of an adjustment part driven thereby, and on the motor speed, using the frequency (ripple frequency) of the current ripple signal. However, in particular high-frequency interference signals are superimposed on the motor signal and therefore the useful signal containing the current ripple. Comparatively low-frequency interference which has an effect on the profile of the DC signal component on which the current ripple is superimposed and can be attributed substantially to the force or torque profile of the motor, which fluctuates in operationally dependent fashion, also needs to be taken into consideration in the evaluation of the current ripple.
In a circuit arrangement for generating a speed-proportional pulse train in DC commutator motors which is known from DD 254 254 A1, a motor signal which is tapped off at a current-to-voltage converter in the form of a nonreactive resistance, is supplied to a voltage-controlled high-pass filter and to a voltage-controlled low-pass filter, connected downstream, for realizing a narrow-band bandpass filter, with a speed-proportional pulse train being generated at the output thereof.
It is furthermore known from U.S. Pat. No. 4,924,166 to use, as controllable filter, a phase locked loop with a phase comparator and a controllable oscillator (VCO) connected downstream thereof via a low-pass filter to process the current ripple generated in the armature current of a DC motor. On the input side, the motor signal filtered using a low-pass filter is supplied to the phase comparator, while the oscillator thereof is driven by a signal derived from the electromotive force (EMF). For this purpose, the armature current signal tapped off at a shunt is supplied directly via a low-pass filter to the signal input of the phase locked loop, while the control signal for the oscillator is supplied to the phase locked loop from the difference between the motor voltage and the product of the armature resistance and the armature current as so-called back-emf signal (back-electromotive force).
It is also known from DE 195 11 307 C1 to drive an adjustable bandpass filter using the back-emf which is determined from the detected motor voltage and the detected armature current. Otherwise, in this known method, the frequency filtering is adjusted by estimating the present useful frequency (ripple frequency) of the armature current signal in such a way that the passband comprises the useful frequency and is above and/or below the interference frequencies. The relative extreme values (minimum and maximum evaluation) are determined from the signal filtered in this way.
Even in a method for determining the number of motor revolutions in electric motors from current ripple which is known from DE 198 34 108 A1, the motor current signal is first supplied to a low-pass filter in order to eliminate high-frequency interference. The current ripple is in turn determined using calculated differential values between successive minimum and maximum values of the motor current signal filtered using a low-pass filter.
In a circuit arrangement for detecting a speed-proportional pulse train for a DC commutator motor as known from EP 0 579 015 B1, a frequency-selective filter with a limit frequency which varies corresponding to the motor speed is provided. In this case, the lower frequency, given virtually unchanged motor speed, is markedly below, but, in the event of a change in the speed, is approximately in the range of the pulse train. It should thus be possible to achieve temporarily a differentiating response of the bandpass filter during changes in speed and therefore also to precisely determine the rotary position of the DC commutator motor during these changes.
In order to configure a frequency filter variably, it is known from EP 1 037 052 B1 (DE 600 05 727 T2), to use an active high-pass filter with switched capacitances (switched-capacitor filter, CR filter) in order to eliminate interference from the motor signal and to determine the speed from the ripple of a DC motor. For this purpose, capacitances are switched by means of a clock signal, with the result that the filter cut-off frequency changes and a downstream circuit generates a ripple pulse train, which also acts as a basis for the generation of the clock signal.